As with other situations, I tend to hold back and contribute less than I'd like to, but I always felt as able to have a conversation with someone in the office as anywhere else. In some respects it's easier practicing conversation at work as you're usually lumped together with strangers and need to communicate as part of the job. That kind of breaks the ice in itself and you can sometimes relax into more casual conversation during down-time without having to worry about explaining yourself. It depends on the culture of course, but everyone I spoke to at my old workplace was very patient and accomodating.