Fast-paced and intense. In general, everyone is fairly friendly and welcoming, but they are very driven to complete projects holistically (complete projects, not just their specific contribution.) If you do not enjoy intense, challenging projects you will probably not enjoy working at Tesla.
A day in the life at Tesla meant going to work, sitting at my desk all day doing development. There was a standup meeting from 10-10:15, and a meeting with my customer in Engineering about 1-2x per week, but in general there were very few meetings, it was mostly development time. Also for lunch, they do not really reach out to have you join them, and everyone brings their lunch, so if you want to eat with people, you should make friends with the other interns and eat with them.
Tesla has a unique company culture. People love what they are working towards and are willing to work a lot. This translates to a somewhat frantic, innovative environment where people are all working extremely hard to achieve the mission of the company. Extremely driven.