What's the difference in meaning between a "monologue" and a "soliloquy"?
A soliloquy is a character making a speech, usually when alone. To get it off his chest, and of course so the audience know what he's thinking. Most famously, Shakespeare's plays are full of them, as characters lament their fates and plot their revenge. It's a speech out loud to oneself.
Soliloquys ARE diegetic. That means the character can hear himself speak. You're not hearing their thoughts, the character is really talking. In Romeo and Juliet, Juliet gives a soliloquy that's overheard. She still intended it as a soliloquy, it's addressed to herself, that's the point. Soliloquys are usually given when the character is alone, but it's not necessary, a character can give a soliloquy in front of other people, who would be able to hear him. They'd probably think he was a little wierd though, depending on the kind of characters they are.
A monologue is just a character talking at length. Either giving a speech, or just boring people. Sometimes used as a criticism when a character's speech goes on too long and the audience, or critic, is bored. A monologue can also be heard by the characters. It can be aimed at any amount of characters who are there to hear it, one man or a crowded hall.
The difference is who it's aimed at. A soliloquy is a self-aimed monologue.
There's also an "aside", where a character tells what he's thinking, or how he feels, to the audience. Asides are not heard by the characters of the play, only the audience hear them. For their duration, the character acknowledges the audience, then forgets them, when back in the world of the play. Though it is the character who speaks, not the actor. It's "crossing the fourth wall", so has to be used skilfully. Iago in Othello makes a lot of asides. They are partly diegetic, only Iago and the audience hears them.
For sound, even though you didn't ask, diegetic sound is sound the characters hear, ie a bird singing, or if someone in the play or movie turns on a radio. Extra-diegetic sound refers to sound the characters can't hear, eg the theme music of the film, or someone coughing in the audience. The actor can of course hear it, his character can't. Or in Star Wars, none of the characters can hear John Williams's excellent musical score. But they can hear the music of the Cantina band, Figrin D'an and the Modal Nodes. The band is "real" to the characters, but the orchestral score only exists in "our" world, the real world.
The difference between things the characters are aware of, and things that only the audience see or hear, or know, is an important concept in drama. Crops up a lot.
Finally, as Ankur Bannerjee points out, people give monologues in real life. Either as a speech, an argument, or just someone who likes to talk too much! It would be very strange if someone gave a soliloquy in real life! Especially if you saw them!
monologue occurs when a character talks to himself out loud when no other characters are either on stage or within hearing distance whereas soliloquy is a monologue restricted to drama that actors speak only when alone, or when they believe they are alone
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