As I see it, the power and characteristics of an Elohim's revelatory (or apocalyptic) form is dependent on the interplay between her primary lore, her Faith, and her Torment. Angels weren't defined by their jobs so much as they were their jobs, as evidenced by the revelatory form reflecting the character's primary Lore (or is it the other way around?). A fallen angel with weak Faith lacks the spiritual strength to manifest her Celestial form fully, while those with more Faith are more confident, more angelic, more there, which is evident in her more glorious (or terrible) visage. And while Torment is a great source of frustration for demons, and cripples their struggle for redemption and understanding, it's also a source of dark and terrible strength -- especially for those monsters that care only for reveling in revenge or destruction.
When players create demon characters, I allow a considerable amount of customization for those characters, going so far as to allow creative players to develop their own traits and advantages (within reason, of course). I use the listed forms for the houses as a base "theme", and allow players to build upon that. While all Anshar share similarities, for example, each Scourge's distinct nature shines through as the Fallen awakens more of her angelic nature. The rules below grant fewer revelatory characteristics to beginning characters, but all characters receive Attribute bonuses, which helps make up for this. In addition, the character gain access to more revelatory characteristics and Attribute dots as traits increase during play, making the apocalyptic forms of experienced demons potentially far more powerful than the stock visages from the book. This method of determining one's revelatory form is more complex than just picking a form, but the math is no more involved or time-consuming than one would perform in a turn of combat, and probably less complicated than the point-based method in the Demon Players Guide. (However, see the box way below for a hybrid point-based system.)

Selective Manifestation
Even if you don't use any of my other rules, below, you might like this handy clarification of the "partial apocalyptic form" rules in the core book (pg. 170). (At one time the following rule was a Merit listed in Memories of Glory, Broken Wings; however, after consideration -- prompted by feedback from a helpful reader -- I've decided to apply this option to all demon characters in my chronicles.)
Any Fallen character can briefly channel one of the benefits of his apocalyptic visage through his host's mortal body without causing Revelation, as long as the power is subtle. This effect only lasts for a few moments, but often that's all that's needed. A successful Faith roll (difficulty 7) allows a character to apply a single revelatory characteristic or benefit he has to his mortal form; this lasts for a number of turns equal to the successes on the Faith roll, though multiple Faith rolls may be made to keep the enhancement active. A botch drains a point of Faith from the character and renders this ability unusable for the next 24 hours. Selective manifestation allows a character to perform certain extraordinary tasks or gain useful traits without having to employ his full visage (and risk Revelation or undue attention); popular enhancements include reduced Ability difficulties, enhanced Attributes, Extra Actions, or apocalyptic soaking benefits. Certain traits (like Huge Size, Shroud of Flames and Quills) can be channeled into the host's body, but are blatant effects and will trigger Revelation in witnesses.

Balance Considerations My rules give free bonuses to Attributes, which players are bound to love. And so do certain revelatory characteristics, such as Huge Size and Inhuman Allure. No, Storytellers, this isn't designed to break your game. In the spirit of fairness, sanity and game balance, I recommend that the listed bonuses not be applied if using my house rules for Lore-based stat bonuses. Storytellers have two options, here. The first and easiest is to assume the above covers Attribute bonuses, and that's that; players can't buy any more bonuses. Another way is a middle path, of sorts: Improved Attributes is a common form ability, costs 3 pts. (or 2 pts. if bought Tormented), and grant three bonus dots which may be spread among the fallen's Attributes as the player desires. This ability may not be taken more than once. This standardization serves to iron out uneven bonuses (compare the bonuses of the Dagan and Zaltu visages for an example of this disparity), and prevent revelatory stats from climbing too high.

Enhanced Attributes
All fallen receive Attribute bonuses in their revelatory forms. Add the character's Faith to the number of dots the character has in her primary lore (not in every lore!), and subtract two: this is the number of bonus Attribute dots that are added in when she is in revelatory form (even when tormented). Whenever a character's Faith or primary lore increases the player chooses where that dot is assigned. Attribute bonuses are fixed, and can't be reassigned or moved around.
Each House has affinities for particular Attributes, and half of these bonus dots (rounded down) must be assigned to those Attributes. The other dots may be assigned as desired. Devourers and Malefactors invariably have high Physical Attributes; Devils and Defilers emphasize Social Attributes; and enhanced Mental Attributes fit the intellectual Fiends, Scourges and Slayers.
Storytellers might be wise to restrict characters from dumping all their points into one or two Attributes; a maximum bonus of +4 for any one Attribute isn't unreasonable, though Storytellers not afraid of epic games may allow bonuses to climb as high as the Celestial's Faith. Also, no character can have more than ten dots of bonuses, total, in any one Attribute category.

Revelatory Characteristics
Add a character's Faith to the number of of dots she has in her primary lore, and then divide by two (rounding down); this is the number of revelatory characteristics a fallen has. A fallen always has one revelatory characteristic associated with her visage, and half her characteristics (rounded down) must be chosen from her visage or those visages associated with her house; remaining "slots" may be filled with revelatory characteristics from other visage (or custom powers). As Faith and primary lore increase, a Sebettu may develop more characteristics, chosen with the above limitations; if enough permanent Faith is lost, the character loses access to the associated trait until Faith is recovered. In certain cases, a Storyteller may choose to waive the above guidelines or even allow characteristics normally only available for a high-Torment visage, if he feels other characteristics better reflect the player's concept.
Tormented Characteristics: To determine how many high-Torment characteristics a fallen has, add her Torment to the number of of dots she has in her primary lore, and then divide by two (round down). These characteristics don't appear unless the character's Torment is high enough to manifest them (as per the rules on pg. 170 of the core rulebook), or the character chooses to manifest them (which inflicts temporary Torment). As with normal revelatory characteristics, half the high-Torment characteristics should be picked from her visage or house list, though the others are may be picked from other lists or made up. Permanent Torment increases have the potential to grant more high-Torment characteristics, while loss of Torment reduces the strength available to the apocalyptic form proportionately (a warning to would-be "nice demons" that decrease their Torment).

Birth of a Visage
Elizabeth is creating a Malefactor Antu, Rivahl, with Lore of Paths ●●, Lore of the Fundament ●, a beginning Faith of 4, and Torment 3. She gets four dots to assign to her Attributes ([Faith 4 + Lore of Paths ●●] -2 = 4), and chooses +1 Strength, +2 Stamina, and +1 Wits.
Now on to revelatory characteristics. Rivahl has three characteristics ([Faith 4 + Lore of Paths ●●] / 2 = 3). She chooses Dead Reckoning, as it's her favorite one from the "base" Antu visage. Next, she chooses Pass Without Trace (another Antu trait, and one that normally high-Torment). For the last characteristic, Liz submits a custom one that allows the Antu to run at three times normal speed. An interesting mix. Liz explains that her angel's role during Creation was to work with Wild House to carve out landscape where the deepest wilderness would grow. In the war against Heaven, Rivahl employed her skills as a scout and skirmisher. Appearance-wise, she has the look of a weathered traveler, but with deep green, grass-like hair and shifting patterns on her skin that resemble the shadows of foliage. The Storyteller approves, but he tells Liz her character should invest in Lores of the Wild later on to reflect her connection with the wilderness. Liz agrees (she was going to do that, anyway), and eagerly begins picking her high-Torment characteristics.
Liz's Malefactor only has two high-Torment characteristics ([Torment 3 + Lore of Paths ●●] / 2 = 2.5, rounded down to 2); however, she sees her character's Torment increasing soon, so she doesn't fret over this. The out-of-house characteristic Cloak of Shadows immediately appeals to her, as she imagines the shadows on the Tormented angel's skin growing to envelop her. She picks Relentless from her visage's list, which complements her Fast Running trait nicely.
Rivahl's visage is now complete, though its capabilities will certainly grow as the character progresses.

Mix n' Match: the Point-Based Method You might be saying right about now, "I like your basic idea, here, dude, but I like the flexibility of the point-based system in the Demon Players Guide when building apocalyptic forms." Fret not, I like the point system too. However, I can't assume everyone has that book, or uses that system. Basing these rules on the point spread would make them unusable for those Storytellers. But I don't want to alienate those of you who, like me, prefer a point spread to buy your characteristics. So in the interests of making my rules appeal to as many Storytellers as possible, I've created a hybrid system that offers the flexibility of the point system while taking into account the character's Faith, Torment and primary Lore ratings. To determine the number of points a character has to spend on standard revelatory characteristics, simply add the character's Faith and her primary lore rating together. For points to spend on Tormented characteristics, add the demon's Torment to her primary lore. Then, use the rules from the Demon Players Guide, following the point costs and standard restrictions in there -- though you needn't meet the required number of powers (four normal, four Tormented), as you probably won't have that many points. Be sure to add the trait-based Attribute bonuses, above. As Faith, Torment and Lore increase, you gain more points to spend on revelatory traits (though decreases in Torment can have the opposite effect). Simple, no? Want one of those nice, italicized illustrations? Sure, I can do that: Using the example above, let's assuming Elizabeth is playing under a Storyteller that used the point-based spread for creating visages. Liz has 6 points to spend on Rivahl's standard revelatory traits, and 5 for Tormented characteristics. She still receives a total of four bonus Attribute dots.