The approach of this guide is to modify an internal settings file. First, you need to navigate to If you don't know where that is, you can use the Steam Client to help you.

1. Right click the game in your Steam Library and click on "Properties"
   
2. In the window that was opened, go to the "Installed Files" category and click the "Browse..." button on the top right of the window
   
3. Clicking the button in Step 2 should've opened a file explorer window in the folder Warhammer 40,000 Rogue Trader folder

After opening the explorer in the previous step, you now want to open startup.json file, If (for some reason) file extensions are hidden from you it's simply called startup.
Use a simple text editor to open a file (as you would open any other .txt file).

The exact contents of this file may differ, but in general they should be similar:

• Opening {
  
• Pairs of "SomeName":SomeValue
  
• Closing }

As an example, here is my fiile:

What you want to do is go to just before the closing } and

• If you want to use a Controller, insert:
  
  , "ForceControllerMode":"gamepad"
  Which in my case makes the file look like this:
  
  
• If you want to use Mouse and Keyboard, insert:
  
  , "ForceControllerMode":"mouse"
  Which in my case makes the file look like this:
  

Now you just need to save the file and close it. After this, simply starting the game should now force it to pick your input method of choice

Note: this probably needs to be done every time the game updates!

Open Steam\steamapps\common\Warhammer 40,000 Rogue Trader\startup.json and add the Key-Value Pair "ForceControllerMode":"mouse" or "ForceControllerMode":"gamepad" to the collection.