Shadows of Thassilon Character Creation
For the most part, Shadows of Thassilon uses the standard Pathfinder character creation rules.
As mentioned under Campaign Basics, there are two Player's Guides for Rise of the Runelords, one for the original 2007 edition of the adventure path and the other for the 2012 Anniversary Edition. Both contain quite different information and I believe it's worth having a look at both, though much of the rules information presented in the 2007 edition has been superseded by subsequent Pathfinder publications.
Both of the above are free downloads, but if you don't have a Paizo account and don't wish to register one, please let me know via PM and I'll see about getting a copy to you.
Most of Paizo's adventure paths are written primarily to accommodate relatively heroic characters of good or neutral alignments, and Rise of the Runelords is no exception. I am hesitant to allow evil characters as they tend toward disruptive behaviours and distasteful character concepts; if you have an idea for a character that only works with an evil alignment, I'm happy to give it consideration but I can't promise I'll accept it.
Roll 4d6, discard the lowest die result, and add the three remaining results together. Record this total and repeat the process until six numbers are generated. Assign these totals to your ability scores as you see fit.
The ability score array used by NPCs with heroic classes (15, 14, 13, 12, 10, 8) generates a character with ability score modifiers totalling +5 before racial adjustments. Rerolls will be allowed at the GM's discretion, in the case of low ability score totals or arrays that the player finds unappealing.
The races presented in the Core Rulebook (dwarf, elf, gnome, half-elf, half-orc, halfling, and human) are available for play as usual. Other races from other sources are available only at the GM's discretion.
The classes presented in the Core Rulebook are available for play as usual; the classes presented in the Advanced Player's Guide, with the exception of the summoner, are also available for play. Other classes from other sources are available only at the GM's discretion. Consider also the following guidance.
Summoners are not available for play. There are too many ways in which their particular combination of abilities (an extra set of actions every turn from their eidolon, strong buff spells, and increased access to summoned monsters through their summon monster spell-like ability) can lead to situations that are the opposite of fun for the group as a whole.
Cavalier and Druid
Druids share many of the same qualities as summoners (a companion that grants an extra set of actions, strong buff spells and abilities, and an increased focus on summoning spells compared to other spellcasters) but have a stronger history within the game that I'm not so willing to discard, and have access to alternative choices or class features that don't necessarily lead to the same problematic situations.
The cavalier presents a different set of problems, namely that cramped dungeons with five-foot-wide corridors appear extensively in the early modules of the adventure path, limiting the opportunities for Medium-sized cavaliers on Large-sized mounts to bring their most defining class feature into play. A Small-sized cavalier will more easily find opportunities to shine, earlier in the campaign, as will a cavalier who selects options that de-emphasise mounted combat; on the other hand, I appreciate that much of the appeal in this class lies in playing the archetypal armoured knight on horseback. I don't know that there's an easy solution to this; it's simply a point I feel is worth keeping in mind.
Necromancer (wizard arcane school)
Necromancers who create undead minions combine the problems of evil characters and summoners or druids, committing evil acts to clutter the battlefield with additional combatants taking extra actions. Thus, necromancers who create or command undead are not available for play.
The feats presented in the Core Rulebook, the Advanced Player's Guide, and the Inner Sea World Guide are available for play as usual, with the exceptions and guidance noted below.
The feats Additional Traits (APG p. 150) and Leadership (CRB p. 129-130) are not available. Note that characters in Shadows of Thassilon have three traits rather than the standard two.
Item Creation Feats
Item creation feats are available as usual. Please note that items created by PCs should be intended for use by PCs, rather than as part of a money-making venture; I'm aware that the magic trait Hedge Magician (APG p. 329) and the campaign trait Merchant Family (RotRLPG p. 4) interact in particular ways with the creation and sale of magic items by PCs, and will modify these traits if necessary to prevent abuse.
Characters in Shadows of Thassilon have three traits, rather than the two specified as standard in the Advanced Player's Guide (p. 236). One must be a campaign trait, selected from the Rise of the Runelords Player's Guide. The other two can be selected from those presented in the Advanced Player's Guide, following its guidelines for trait selections and excluding the sample campaign traits therein.
Note that, in case it's not clear, all of the benefits of the campaign trait Giant Slayer apply only against creatures of the giant subtype. Consider the latter portion of the trait to read as follows: “Against creatures of the giant subtype, you gain a +1 trait bonus on Bluff, Perception, and Sense Motive checks and +1 trait bonus on attack rolls and damage rolls.”