When do I use an en dash vs an em dash? It’s a tricky question and appears to be as related to fashion as to strict rules of grammar.

It appears, in recent times, the em dash (—) has fallen out of favour in the UK, and is primarily used only in dialogue. It is not surrounded by spaces.

So:

Em dash:

‘It is with shame that I am forced to admit the truth—I was mis-using the em dash all along.’

En dash:

Molly was forced to admit the truth–she had been mis-using the em dash all along.

Both examples represent a break in the sentence, so capitalising the following word is not correct.

Occasionally writers use em dashes to denote dialogue:

—I was mis-using the em dash all along.

Whatever the circumstance, a space is not required.