Problem with dialogue

I am about midway through a novel with Scrivener, and I realizes that i has been a bit careless about formatting.
Usually, I write in the scripwriting form, maybe this is why i have been careless (ouh, novel, no real format, what a relief I thought it was!)
All my dialogues seem to be formatted in a strange way, with a big, a huge right margin.
I just want them to use the same formatting as the descriptions, but it seems that I have to MANUALLY correct each line, which is not my idea of fun, or writing!
Maybe the program interpreted the “-” at the beginning of each line as a symbol for a list? I do not know.
Is there any way to correct this rapidly?

(the novel is written in French, i do not know if it is relevant)

I’m assuming you want all the paragraphs in the document to be formatted the same? (That is, there’s not any type of paragraph other than “dialogue” and “description”.) You can standardise a document’s formatting in the editor using Documents > Convert > Formatting to Default Text Style, and you can even select multiple documents in the binder and convert them all at once. To set your default formatting before converting, go to the Editor tab of Tools > Options… and use the format bar and ruler to adjust the sample text. (The blue A button on the left of the format bar opens the font options.) Click OK to apply the changes and close the Options.

Thanks for the quick answer.
A question before proceeding:
will the italics inside the text be swept away when I apply the default style?

Nope. Go ahead!

Character formatting (bold, italic, underline, etc.) won’t be affected by the conversion provided that the default font has italic and bold variants. If you’re worried or going to be converting a lot, you can easily make a backup of the whole project first via File > Back Up > Back Up To… so you have something to roll back to if you’re not happy with the changes.