I feel like I should point out, there's really 2 different kinds of scuttles, and it sounds like everyone here is talking about a "modern" scuttle.

I use an old antique scuttle I picked up on ebay, which is quite different in design and use.

First of all, the spout is actually big enough that you dip your brush into the hot water reservoir to wet it. Second, there are holes in the soap dish up top. It's only meant to be a convenient place to set your shave soap, not a place to build lather, as all the lather would get forced through the holes, and down into the hot water reservoir. This also makes it useless for cremes.

The way I use it, is I'll put a kettle full of water on the stove when I get in the shower. If I set my stove to medium-high, the kettle starts to sing about the same time I finish toweling off. (Did I mention that I don't own a microwave?)

I use the boiling water from the kettle to fill the reservoir of my scuttle.

Dip the brush into the reservoir, tap once to shake off grossly excessive water, and twist/pump the brush on top of the soap to load it. I make no attempt to build lather on top of he soap at all.

I then lather on my face. Boiling water in the brush, plus slightly above room temp soap balances out to a piping hot lather brush on my face. Also, using good quality shave soap, like TGQ, I rarely have to load more soap on my brush for 2nd/3rd pass. I just dip the very tip of the brush back into the boiling water. It heats the soap in the brush back up, and adds a little more liquid. Sometimes for the 3rd pass, I'll reload just a little more soap, but usually it's about 3 seconds of loading. No more.