Cheers muchly. I wrote 99% of the dialogue, however the basic story had a fair few people's input into it (i.e. the existence of Chase, Frank, Dunby, Rex, Chan, Vinnie and Blackwell and their basic roles), although eventually it was the project's creative lead and I who dealt with most of the problem-solving in the story as the gameplay changes affected it.
I also had an editor, Andy Walsh who was utterly invaluable in making sure it all made sense and communicated the objectives to the player regularly, which initially involved a BIG re-write. He reminded me to put jokes in when I forgot to and gave me loads of really useful tips on writing. This was only the second game I'd ever written the script for, really, after LEGO Harry Potter Years 1-4 DS (I'm not counting Dragontales: Dragonseek - ugh!) so his constant feedback, help and advice was invaluable.
The gameplay dictated the script for about 90% of the development - which is the correct way around, IMHO. The way the story unfolded was completely down to the order in which we wanted you to get new abilities, and so those new abilities affected when you met new characters and who they were, etc. As a result, some characters only existed for specific gameplay reasons. For instance, Ellie Phillips was written in purely so we had a single character who could remind you of objectives at any time (she also exists for logistical development reasons), although her role expanded as time went on.
The process of writing was pretty iterative in the sense of lines changing ALL the time as the game did likewise. Day to day I'd be either writing, re-writing, going to voice recording sessions in order to make sure the lines were read as intended, processing those sound files when they came in (and then the audio dept would do a bunch more processing), writing tools to compile the audio, sending stuff off for lip-syncing, writing game scripts to play the audio (along with another designer called Ben), setting up the cameras for the in-game scenes (not the proper cutscenes, that was done by a whole other department of talented animators), etc, etc. There was plenty of variety. Truth be told I'd have been happier purely writing the script and the compilation tools, but that's because I'm very lazy.