In the aftermath of the 2015 attack on the Thalys, it was publicised that security was going to be introduced for the Thalys. Where is this? Does this affect all stations at which the Thalys calls?
As far as I can tell, security has only been introduced at the two primary stations it operates from in France: Paris Gare du Nord and Lille Europe; this doesn't seem to have any affect on the seasonal services to Marseille and Bourg-Saint-Maurice, unlike the Eurostar's seasonal services.
This SNCF press release covers the detail (in English!). In short, the security consists of barriered platforms with a ticket check and X-ray of luggage required to access the platforms. The platform is announced twenty minutes prior to departure to allow everyone time to pass through this, and this is the recommended time to arrive at the station.