The following script for Tampermonkey (Chrome) or Greasemonkey (Firefox) will automatically expands all comments which are normally hidden behind the "show ... more comments" link. 

[![enter image description here][1]][1]
    
The code is based on the [fantastic answer from Brock Adams][2] - all praise should be his! 

    // ==UserScript==
    // @name        StackExchange, Expand All Comments
    // @author       Taken from https://webapps.stackexchange.com/a/119439/192282
    // @include      https://*stackoverflow.com/*
    // @include      https://*serverfault.com/*
    // @include      https://*superuser.com/*
    // @include      https://*askubuntu.com/*
    // @include      https://*mathoverflow.net/*
    // @include      https://*.stackexchange.com/*
    // @grant       none
    // ==/UserScript==
    
    try {
    //--- This is a utility function that SE currently provides on its pages.
    StackExchange.ready (function () {
    $('.js-show-link').trigger ('click');
    } );
    }
    catch (e) {
    console.warn ("***Userscript error: StackExchange.ready() is no longer defined!");
    }


  [1]: https://i.sstatic.net/NsQWL.png
  [2]: https://webapps.stackexchange.com/a/119439/192282