A wider aperture lens is the easiest option.
If you like doing things the hard way, a few other options come to mind:
1. Your software should have some blur options. Tweaking these settings and playing with thresholds might work. If you're after some free software to do this, have a look at Davinci Resolve.
2. If you want to get more creative, you could film the background with out the actors. Then film the actors. In post, you merge the two, mask the actors on their layer and blur the background only layer. I find masking really painful, but in theory and with enough patience, this should work.
3. Film your actors in front of a green screen. Then film the background. Blur the background as much as you want to and substitute it for the green screen. Most software should do this reasonably well.