How can I get the audio of only the dialogue of a movie, without any sound effects or music? I really need it for a fan edit.

Whether you can afford this, or even get permission, is another matter. But it is guaranteed to work perfectly.

But much dialog isn't mixed in the center in stereo. It is offset, to set the sound stage left and right.

This works well to separate sounds which are repetitive, like the background noise on a street from dialog, sounds like a steady electric hum, engine noise like a jet or vacuum cleaner, etc.

Taylor Swift and Travis Kelce Wear Matching Outfits For Their First Public Appearance in Months - instyle.com

The cheapest method uses simple audio frequency filters, to reduce parts of the sound spectrum. This can make the dialog louder relative to the other sounds, but won't eliminate them.

The simpler surround sound formats, 7.1 and 5.1, have a single monophonic center track. In any audio editor that can separate these tracks, you can easily get just the center track. If all of the dialog is mixed there, without any sound effects or background music, this will work well. But not all mixes do that, especially if some dialog is intended to come from the sides of the screen.

Get the original master audio track from the movie production. They will have that for any recently produced movie from the surround sound era. Audio is recorded in many separate tracks, and dialog routinely uses many tracks - sometimes multiple tracks per actor.

Switch, Xbox, and the portable future of games - The Verge

That leaves other means to reduce the sounds and extract the dialog. Modern AI audio processing uses filters and pattern recognition to identify elements in the sound.

This process can also extract only the center. If you're lucky, you'll get mostly vocals and dialog.

Beyond that, it isn't easy, and won't ever completely eliminate the music and background sounds.

Which type of physical cable has fastest transmission speed?

In a stereo mix, the monophonic elements will often be balanced in the center, equal in both tracks. A trivial vocal removal effect inverts the phase of one track, then adds them together to produce a monophonic version of each track with the monophonic center reduced.

It's not that great at separation of music, vocals, and random sound effects.

Back to the most perfect, but potentially most expensive method.

Why do flat-Earthers think the Earth is flat?

Let's go in reverse order from easiest and most expensive to simple, cheap, and low quality. Dolby Atmos audio has separate tracks for all sound sources. If you can afford a proper Dolby audio and video editing software suite, you can extract each of these sounds from the mix.