A musician is an instrumentalist who creates and/or performs music. This could be live on a stage or in a recording studio, either as a soloist of in a group. Musicians usually specialize in one type of music, but some branch out into others to expand their range.
Most musicians activities will be similar regardless of what instrument they play or the type of job they have. General tasks for a musician can include;
Performing on stage and participating in recording sessions, Practicing regularly,
Preparing for auditions,
Preparing for and attending rehearsals.
Learning required music.
A Musician can work closely with; 
Actors. Fellow actors may be on stage while the Musician is playing, this means they will share rehearsals together and generally performers spend time backstage with each other, especially if they are touring with each other. They may also give each other feedback, and constructive criticisms which will help improve the individuals performance which will boost the whole shows performance.
Dancers/Singers. If the Musician is assisting in providing a backing track for a dance piece or singing piece then the musician and performers will need to work closely together to construct their performance and rehears it to get it up to a high quality performance.
Musical directors. If a show or production features live music it will most probably have a musical director or someone of that sort who knows a lot about music and how to adjust it so it can be at it's best during a show. A musician will have meetings and one-on-one sessions with the musical director to work on music pieces, receive feedback and check that the musician is keeping on target. The musical director will also pass on ideas or problems that have risen from any meetings between directors and producers.
An actor communicates a character and/or situations to an audience through speech, movement and body language. It usually involves interpreting the work of a writer, normally with the help and instruction of a director, although they may devise their own character or improvise with a character as a reaction to the audiences reactions.
An actors work varies enormously, from live stage performances, community theater, soap operas, radio work and film and TV parts.
Typical work activities can include;
Liaising with an agent,
Job seeking and networking,
Preparing for and attending auditions,
Learning lines and rehearsing,
Research for a part,
Performing for a live audience,
Performing in a studio or 'on location film' for film, television, internet and radio broadcast, Doing 'voice-overs' for advertisements or recording audiobooks, Liaising with venue managers and accommodation providers.
An Actor/Actress can work closely with; 
The Director. The Director will be the one who instructs the performers on what kind of thing he/she wants. If the Director doesn't like a piece then he shall talk to the Actor and they will have to adapt it. The Director likes to keep updated on the performers and will often have meetings with them to check their progress. If an Actor has any ideas or inquiries they can talk with the Director about them.
Musicians/Singers/Dancers. Fellow performers may be on stage while the Actor is also performing, this means they will share rehearsals together and generally performers spend time backstage with each other, especially if they are touring with each other. They may also give each other feedback, and constructive criticisms which will help improve the individuals performance which will boost the whole shows performance.
Script Writer. The script writer will be the person who has the image of a character or scene in their head while writing the script. The script writer would discuss the characters or scene with the Actor to make sure that it follows what they...