During a soliloquy, only the character making the speech is present onstage, while during an aside, others characters may be present onstage.
Explanation:
A soliloquy is a device often used in the theater where a character speaks to him or herself and relates certain thoughts or feelings of an honest nature, and by which the audience learns the contents of these thoughts. Usually during a soliloquy the character is alone on stage. An aside is usually shorter and it usually seems like the character is addressing the audience directly referencing also thoughts of an intimate nature. During an aside it is common for other characters to be on stage as well, but the audience is to understand that the other characters cannot hear the character giving the aside.
