Spying on someone else's console

The standard composition of FreeBSD includes a great watch program (8), which allows you to spy on someone else's console.

All it needs for this is the snp module, which is enough to be assembled from surts and the program itself will load it:
# cd /usr/src/sys/modules/snp/
# make && make install

Now select the target:
# w
9:16PM up 55 days, 52 mins, 2 users, load averages: 0.08, 0.06, 0.00
USER TTY FROM LOGIN@ IDLE WHAT
nlo1 p0 nlo.habr.ru 9:06PM - w
nlo2 p1 nlo.habr.ru 9:16PM - -bash (bash)

And spy on an interesting console to us (TTY).
# watch p1

The question is how to spy on someone else's console in Linux?
ps
The question is not for the sake of hollywood, but I really wonder how this can be done.