Einstein was a theoretical physicist as apposed to an experimental one. People like Einstein generally work with a pen and paper, or chalkboard. Assistants are rare, although exchanging ideas with peers and publishing their work is common. Experimental physicists design experiments, and collect data and often have many assistants. Some cross over into theoretic physics, some to engineering. Usually theoretical physicists don't cross over to other fields. Usually both theoretical and experimental physicists have teaching resonsibilities, but this varies.