Direct fire mortars are the most effective way for suppression and killing enemy teams with mortars. Having your mortar teams numbered for quick access will also assist in hitting your opponent fast. In the video below you will see how fast I can bring down fire on out in the open running troops. The enemy team is wiped out. I assigned the mortar team 1 by pressing CRTL+1. The input sequence of my firing is 1, v, left click, m.
Direct fire has one huge disadvantage. The team will give their position away either by rifle fire or the graphic of the mortar firing. In some versions of Close Combat the supporting members of the team will engage with their rifles. In the direct fire role you must be prepared to defend your team, they are too valuable to lose.
81mm mortar teams are mobile but move very slowly and have a long setup time. Using these teams for direct fire is usually a bad idea. The 60mm mortar teams are much more mobile and have a quicker setup time. They can follow an advance and bring down fire for your assaulters. There are 120mm mortars that are mobile, but moving them is as painful as moving a 17pdr ATG. Other mods have the 120mm mortars set as static units unable to move. If you decide to use direct fire, always ensure you check the mortars minimum firing distance!