Overview
How to get the Joker achievement, quickly and easily, and why call-spamming the Professor doesn’t seem to be doing anything.
How to get the Achievement
The ‘Joker’ achievement is for prank calling the Professor three times via the Phone box outside the University in Chapter One. Any combination of ‘Ima Neediot’ or ‘Quest and Test’ (daytime), or ‘mother’ or ‘lady in a wheelchair’ (evening) will work. You can use the same joke every time or all different ones, and it is fine to skip the dialogue with the Esc key, but there must be a save point between each joke.
Over the course of normal play, you can get this achievement by getting the phone number when you first visit the Professor (Info sign next to door) and making a prank call each time you walk by the phone. Note that the phone will stop working after calling the Professor as Leopold, so make sure the achievement has triggered before that point.
If you are replaying just for the achievement, the most efficient way is to start a new game, go straight for the phone number, then prank call/enter University/leave University three times, using Esc to skip all cutscenes and dialogue.
Making Jokes Count
A lot of people have had difficulty getting ‘Joker’ and there has been a lot of speculation about it.
Making three jokes is not hard, so why wasn’t it working?
The reason why many people have had difficulty with this is that each time the game saves, it will only increment the joke counter by 1 (if you made a joke), no matter how many jokes you made. So staying on the same screen and joking over and over is not very effective. (The same jokes get boring repeated again and again, don’t you know?) Several minutes may go by before an autosave. However, there is a quick and easy way to ensure one! You can guarantee an autosave just by going to another location (ie. back inside the building).
In my testing, I also tried the ‘leather jackets’, ‘box of chocolates’, and ‘Nuno’ interactions (as have been suggested by others). These do not increment the joke counter. Only the Phone box counts.
Happy achievement hunting!