Trying to describe a "typical" ACW appearance is tricky. Appearances varied by time and location. Forage caps appear to be very common for the Army of the Potomac; however, they were less common in the Army of the Cumberland and quite rare in the Army of the Tennessee.
Having lived most of my life in Illinois, I've given a lot of attention to the uniforms of the "Suckers". Based on both photographs and written descriptions, the sack coats were not much in use for most Illinois regiments until 1864. Earlier in the war, Illinois issued shell jackets with epaulettes that buttoned on the shoulder near the neck. Frock coats can also be seen in a number of photographs.
Forage caps are very rare in photos of Illinois troops. Most wore an undecorated version of the Hardee hat. Often the men folded in the crown to lower the overall height of the hat. Forage caps were issued too, but frequently the men complained about them. One sergeant in the 36th Illinois wrote to his mother from outside of Atlanta requesting that she send him a civilian hat to replace his cap, which provided inadequate protection from the sun.