I have been Trauzeuge at a German wedding.
When my wife & I got married just over 19 years ago

two friends of ours were the witnesses & when they got married a year later I was witness (as was also their Danish aunt).
As said above you should show up sober & just have to sign on the dotted line. At our wedding the only (short) speech was by my father-in-law & I certainly didn't give a speech at our friend's wedding.
You get a free meal out of it

In each case the wedding pair organised the Polterabend so witnesses only had to turn up then...