You may be able to slip in and stand or find a seat in the back of the theater; however, some establishments allow only back-of-the-theater sitting or standing until intermission. Before walking down the aisle, take a few moments to adjust to the low light; then scan for a convenient seat. Other patrons will appreciate your not stumbling over them.
If there’s an usher, allow him or her to seat you. For live performances, theaters often have a policy of not letting anyone enter except during a scene change, a break between musical numbers, or intermission. These rules are intended primarily for the benefit of the performers.