I use the pre-mixed stuff that comes in a plastic tub. There are several sizes, but the smallest is probably big enough for most jobs. I find that it sometimes helps to pre-wet the area where you wish to apply the compound; I use "wet" water, same as for scenery, applied with a sprayer bottle. Let the area "sit" for a few minutes after spraying, as it should be more damp than wet. This technique is useful on either wood (plywood and benchwork) or plaster (previously done scenery), as it prevents the surface from drawing too much moisture out of the drywall compound too quickly, a major cause of cracks.
You can also use drywall compound (usually known as "mud") as mortar on brick structures, too. After painting the assembled structure with a suitable flat brick colour, and after the paint has fully dried, use a clean rag over your fingertips to apply the mud all over the brick areas. You don't have to be too neat and you only need enough to fill the mortar lines between the bricks, although it's difficult to not get it everywhere. When you've finished this operation, set it aside to dry; perhaps an hour or two, but even a few days won't hurt, if you don't have time to get back to it right away. Then, using another clean rag, wipe off the excess mud from the face of the bricks, and any other areas where it shouldn't be. This will leave "mortar" between the bricks, and will also tone down your brick colour, giving it a slightly weathered appearance.
The auction building at the Lowbanks Stockyards was painted with Floquil Reefer Orange, then mortar added as outlined above. I didn't bother to seal it with a clear spray before weathering with a wash of dilute PollyScale paint, and, as long as you don't "work" it too much with the brush, the mortar is hardly affected. My technique is to apply the wash, very generously, with a very soft 1" brush at the top of the wall, then let it flow down. With the structure sitting on sheets of newspaper, much of the run-off will be soaked up. You have to keep on top of the drying process, though: as the wash pools around details like window sills, etc., or collects at the bottom of the walls it needs to be gently siphoned off using the corner off a paper towel. This will prevent the formation of unsightly "rings" when everything is dried.
Wayne