The leader will be the highest paid member of the orchestra. He has to decide how the violins will play the music and write in the bowing. He will decide where each member of the violins should sit. He will probably also interview them and offer them the job in the orchestra. He will talk to the conductor about anything the orchestra are not happy about (he may have to remind the conductor when it is time to stop the rehearsal!). If part of the music is marked "solo" then he will play it as a solo while the other violins stop playing.