Players should be inquisitive, pro-active, and Good Samaritan types. That is, I expect the players to display ‘interested involvement.' This means not simply reacting to events as they unfold, but actively investigating leads, seeking clues and following hints. Discussion in-character of such leads, of possible options, and plans of action are important for character development. In fact, I encourage scenes where the characters are merely interacting with each other and NPCs so as to give opportunity to dig into their personalities. There will also, as stated, be action scenes, in some cases initiated or avoided by the characters based on their decisions.
A Note on Writing: As for actual writing ability, I don’t expect everyone to be Shakespeare, but I think it is fair to expect decent punctuation, proper spelling and an effort to carry along the story so that it is enjoyable for everyone. The use of pronouns is important, as well as some effort into breathing life into the character and the setting. A bit of what the character is thinking, some description of his/her mood, body language, demeanor, and grist for another character to interact with. It doesn’t have to be long, though it could be. It’s not about quantity or volume, it’s about quality. The other players should be able to see what you are doing, how you are doing it, and some cinematic license into the reasons and thoughts behind the actions. We should all be mature enough to separate character knowledge from player knowledge, and it tends to enhance the game by making the characters a bit more real. The main caveat here is that etiquette requires that a player never physically engage another player’s character without consent or a chance to react; likewise, a player never has another player’s character react, think, speak or otherwise act without consent. Just focus on your own character and if a reaction from someone else is needed, pause for the reaction.
Use bold in quotes for actual speech, italics for actual thoughts, regular text for everything else.