Though acrylic paint is waterproof once dry, before it dries it is water soluble. This means you can adjust the thickness of the paint by mixing water into it. So you can work with paint as thick as paste or as thin as watercolors, depending on your preferences and what effects you're trying to create in your work. If the paint gets too thin, let it air dry for a few hours, stirring regularly, to let the water evaporate out.
Acrylic paint can dry in as little as 10 minutes. This can create some problems if the work you're doing requires keeping it in a more liquid state for longer while you perfect shapes and mixtures of paint color layers. Minimize this problem by using a moisture-retaining palette to mix your colors, or keep an atomizer spray bottle on hand to spritz the paint every so often.
One of the major benefits of working with acrylic paint is that you can layer the paints to create color effects based around varying degrees of opacity or transparency. The more watered a paint is, the more transparent it will be. If you apply one color of wet paint on top of another, the colors will mix, but if you apply wet paint over dried paint, the colors will layer. Another way to create layering effects is to spray dried acrylic paint with rubbing alcohol; this will cause underlying layers to show through.
Another way to play with the thickness of acrylic paint is to take advantage of the three-dimensional application of thick paint. When you apply paint of a paste consistency, blot with the brush to create a rough texture. When this dries, apply a thinner paint inside the crevices of this shape. You can create a similar effect inside the valleys and lines of a smooth application of thick paint. Wipe away excess paint from the second layer with a damp cloth to leave just the paint in the crevices.
Mix acrylic to a liquid consistency and pour or spray it on the canvas to take advantage of the textures and shapes created by the natural movement of water. Use a spray bottle, pour the paint against the canvas, or splatter-paint by flicking brushes.