All characters progress as fighters, in terms of hit points, to hit rolls, XP, saving throws, etc. They can all use any weapons and armour. (Probably... I am also considering a simple weapon proficiencies system, where each character is proficient in like 4 weapons, and suffers -2 to hit with others.)
What differentiates characters (apart from their randomly determined race and any background / personality traits which the player might imagine) is a choice of talents.
Before going into what the talents do, here are four standard skill rolls which players can make. You will notice these are based on the standard "adventuring actions" of D&D. All skill rolls have a base 2 in 6 chance of success, modified by specified attribute (minimum 1 in 6 chance, maximum 5 in 6).
- Move quietly (DEX) - penalised by 1 per point of AC below 8.
- Search - one turn per attempt. (Note that you stand a better chance of finding things if you describe specifically what you're doing.)
- Climb (DEX) - 30' per roll, penalised by 1 per point of AC below 8.
- Smash stuff (STR) - usually makes a lot of noise.
So, the talents. Each player can choose 3 of the following for his PC. As currently conceived, these are a one-off choice -- it's not possible to learn new talents as a PC advances. Again, many of them are based on a 2 in 6 chance of success, possibly modified by an attribute.
- Lockpicking (DEX) - one turn per attempt.
- Pick pockets (DEX) - target notices on a roll of 6.
- Tracking - make a search roll to find & follow tracks.
- Languages - know additional languages per INT score.
- Climb sheer surfaces (DEX) - automatically succeed at climbing, roll to climb sheer surfaces.
- First aid - spend one turn up to 1 hour after a battle to heal 1d3 hp.
- Move silently (DEX) - automatically succeed at moving quietly, roll to move silently.
- Weapon specialist - +1 to hit with specialised weapon.
- Heavy armour specialist - 25% less encumbrance from metal armour.
- Slayer - +1 to hit & damage against special enemy.
- Tech lore (INT) - one turn per attempt to identify tech items. +2% to use artefacts.
- Arcane lore (INT) - one turn per attempt to identify magical effects or creatures. +1 bonus to learning rituals.
- Awareness - +1 bonus to searching. Only surprised on a 1.
- Poisons (INT) - use poisons safely, one turn per attempt to identify substances.
- Backstab - +4 to hit, double damage.
It's a nice, quick, simple system, but I think it should provide enough choices and variety to make characters interesting.
Next up: rituals! (and that's where Call of Cthulhu will also enter the mix :)