After I applied online from their website, I didn't get any calls until almost after a month from the HR asking if I was ready for the interview rounds starting the day after. I got an email from the Development Team with a programming task. The task was to develop an iGoogle like interface using Javascript, HTML and CSS. I was free to use any javascript library I wanted to. I replied them back within the time limit i.e. 2 days. I got a call from the Development team again the next to next day after submission for the 1st technical round. They were two members on the other side of the phone. The questions were related majority to the assignment I submitted. Basically questions like, why I used JQuery, how else could have you achieved a particular functionality other than how you did etc.
Later they started asking me generic questions about, delegates, functions in pure javascript, about datatypes, using prototype functionality etc.
Basically, if you do your homework properly, you can get through easily. They were mostly casual in the way they communicated and the interview was pretty organized and timed. The work culture at ThougthWorks is quite similar to Google's, at least in the Pune office. Most of the employees are quite young.