Funny story. While brainstorming the Advanced Class Guide, and determining which classes were to appear in it, the investigator almost didn't make the cut.
The class was my favorite from the start. I love detective stories and have been a fan of Doyle's work since first reading The Hound of the Baskervilles all those years ago. I've also been delighted by the numerous reinterpretations of Sherlock Holmes popping up over the decades. For a long time now, I've wanted to cast the shadow of that brilliant investigator and chemist in the light of Pathfinder by fusing the rogue and the alchemist. I knew it wouldn't be easy, but there were fears that the task might be nearly impossible or the class would not find traction with the players.
To our relief, those fears were unjustified.
From the start, a sizable group of players grokked the class. Pathfinder is a combat game, to be sure, but it's also a game of exploration, where players uncover the secrets of their GM's campaign. While some classes are pretty good at exploring story and secrets, their true potential often doesn't ramp up until higher levels. The investigator, through the inspiration mechanic, gives players a leg up from the start.
While the concept of the investigator was well received, that didn't mean the playtest and further development were a walk in the park. The early iteration of the investigator relied on a later-level progression of the rogue's sneak attack to ramp up its combat effectiveness (he can't uncover secrets all the time). Playtests showed us that wasn't good enough. The investigator's ability to increase his combat potential needed something new to highlight his unique take on adventuring. We went back to the drawing board and created studied combat and studied strike. These new mechanics allows the investigator to study his opponent and gain bonuses in combat, until he has studied his foe enough to unleash a damaging, maybe even debilitating, strike. This mechanic kept the investigator true to his theme in and out of combat. The early versions of those abilities were unleashed during the second round of playtesting, and through the great feedback we were able to refine these mechanics into their final form.
Studied Combat (Ex): With a keen eye and calculating mind, an investigator can assess the mettle of his opponent to take advantage of gaps in talent and training. At 4th level, an investigator can use a move action to study a single enemy that he can see. Upon doing so, he adds 1/2 his investigator level as an insight bonus on melee attack rolls and as a bonus on damage rolls against the creature. This effect lasts for a number of rounds equal to his Intelligence modifier (minimum 1) or until he deals damage with a studied strike, whichever comes first. The bonus on damage rolls is precision damage, and is not multiplied on a critical hit.
An investigator can only have one target of studied combat at a time, and once a creature has become the target of an investigator's studied combat, he cannot become the target of the same investigator's studied combat again for 24 hours unless the investigator expends one use of inspiration when taking the move action to use this ability.
Studied Strike (Ex): At 4th level, an investigator can choose to make a studied strike against the target of his studied combat as a free action, upon successfully hitting his studied target with a melee attack, to deal additional damage. The damage is 1d6 at 4th level, and increases by 1d6 for every 2 levels thereafter (to a maximum of 9d6 at 20th level). The damage of studied strike is precision damage and is not multiplied on a critical hit; creatures that are immune to sneak attacks are also immune to studied strike. If the investigator's attack used a weapon that deals nonlethal damage (like a sap, whip, or an unarmed strike), he may choose to have the additional damage from studied strike be nonlethal damage instead of lethal damage. If the investigator chose to make an attack with a lethal weapon instead deal nonlethal damage (with the usual –4 penalty), the studied strike damage may also deal nonlethal damage.
The investigator must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot. An investigator cannot use studied strike against a creature with concealment.
Furthermore, investigator talents allow this class to increase the effectiveness of these two abilities, some of which allow you add debilitating conditions to the studied strike.
Lastly, while the various incarnations of Sherlock Holmes were the inspiration for the investigator, there are many types of investigators floating around the creative consciousness. When designing archetypes, we wanted to cover as many of those bases as space allowed. In the book you'll find the mastermind (an investigator that manipulates via a group of minions), the sleuth (a gumshoe who uses luck instead of inspiration), the spiritualist (a detective who gains insight from the world beyond), and much, much more.
In short, if you have a hankering for playing the smartest guy in the group, we think you'll dig the investigator.