There are several reasons to slow the drying of interior acrylic paints. Very hot, dry air dries paint too quickly, making it sticky and difficult to work with. Many decorative and faux finishes require a longer "working time" and need to be kept wet long enough for you to apply and manipulate the finish. Adding the right retardant to acrylic paint makes application smoother.
Make paint wetter to slow the drying time. Thinning acrylic paint with a little water as a retardant makes it flow on more smoothly, especially over very absorbent surfaces such as fresh drywall or raw wood. You can add up to 1 pint of water per gallon to most acrylic paint--although some formulas should not be thinned, so read the label directions first. In hot, dry conditions, use the painter's trick of floating some ice cubes in the paint. It melts slowly as you work, preventing paint from skinning over in the bucket.
Most paint and hardware stores carry acrylic glazing medium. Typically used for faux painting, glazing medium is clear and can be mixed in any proportion with paint, or used as a varnish. Adding more than 1 quart of glazing medium to 1 gallon of paint will reduce the hide, making the paint a bit translucent. Adding about 1 pint per gallon will not affect the hide much, and effectively retards drying time.
Several brands of acrylic or water-based paint conditioner are available. This is a good option if you are applying the paint with a brush and want to reduce the appearance of brush marks for a very smooth finish. Paint conditioner also slows the drying time of acrylic paint, and you can add 1 quart or more of paint conditioner per gallon of paint without it affecting the final appearance or requiring more coats.