Mmm. The point about spell interruption is good. 3E definitely made it easier to get spells off. Hence, I suspect, why Complete Arcane had those 'screw the mage' feats for martial characters.
The aforementioned 5' step, of course, is the easiest way to do it. Casting defensively in 3E required a Concentration check of DC 15 + the spell level. Like most skill checks, it's easier as you go up in levels -- assuming you're a 10th level caster, you should in theory have 13 ranks (it's a class skill for most casters) plus your Con bonus (let's be picky and say you've only got a +1). So, +14 on the check. If you took the Combat Casting feat, you get +4 to that check.
Assuming a 10th level wizard with +14 on the check, he needs a 6+ on d20 to defensively cast a fifth level spell (his highest level available, at DC 20). To quote the Mandalorian, 'I like those odds.'
Pathfinder 1E was a little better. PF eliminated the Concentration skill, and changed it to d20 + caster level + casting stat modifier. However, the DC got harder (DC 15 + (2x spell level)). Taking that 10th level wizard again, his Concentration check is probably the same -- assuming an 18 Int, he's still got a +14 on the check. But now he needs a 11+ on d20 to cast his fifth level spell defensively. Since PF was a little more generous with feats than 3E, you'd probably see that wizard taking Combat Casting if his buddies kept having trouble with keeping him from being mugged.
It should be noted that a quickened spell (using the Quicken Spell feat) does NOT provoke attacks of opportunity.