There are 10 (hidden) cameos char in the game. The cameos are behind doors or in chests etc and you have to "free" them by destroying these objects.
10 Cameos are and where to find:Episode 01 - Burne
Episode 02 - Irma
Episode 04 - Atilla
Episode 05 - Genghis
Episode 06 - Napoleon and Vernon
Episode 09 - Rasputin
Episode 11 - Dask
Episode 13 - Kala
Episode 14 - Zak
For this achievement, you only have to find the cameos but the cameos still have one more achievement. You have to find them certain things in the game aka collectibles for the cameos.
Task:Burne wants Classic Headlines Collectibles (3)
Irma wants Secret Diaries Collectibles (5)
Vernon wants VHS Tapes Collectibles (5)
Atilla, Genghis, Napoleon, Rasputin wants Disgusting Bugs (8)
Dask, Kala, Zak wants Crystal Shard Collectibles (3)
For completing all tasks you will obtain another achievement (Multitasker - Complete all of the special requests in Story Mode)
I have created a video with all collectibles and cameos:
Timeline:
*** Spoiler - click to reveal ***00:00 - Episode 01
01 - Cameo 1/10 - Burne
02 - Classic Headline 1/3
00:27 - Episode 02
03 - Classic Headline 2/3
04 - Cameo 2/10 - Irma
05 - Secret Diary 1/5
01:05 - Episode 04
06 - Secret Diary 2/5
07 - Cameo 3/10 - Atilla
08 - Disgusting Bug 1/8
09 - Classic Headline 3/3
01:48 - Episode 05 - King Of The Spill
10 - Secret Diary 3/5
11 - Cameo 4/10 - Genghis
12 - Disgusting Bug 2/8
02:18 - Episode 06 - Mall Meltdown
13 - Secret Diary 4/5
14 - VHS Tape 1/5
15 - Cameo 5/10 - Napoleon
16 - Cameo 6/10 - Vernon
17 - Disgusting Bug 3/8
03:00 - Episode 07 - Roof Running Reptiles!
18 - Secret Diary 5/5
19 - VHS Tape 2/5
20 - Disgusting Bug 4/8
03:29 - Episode 09 - Crisis At Coney Island!
21 - Cameo 7/10 - Rasputin
22 - Disgusting Bug 5/8
03:46 - Episode 10 - A Few Screws Loose
23 - Disgusting Bug 6/8
24 - VHS Tape 3/5
04:08 - Episode 11 - Dinosaur Stampede!
25 - VHS Tape 4/5
26 - Disgusting Bug 7/8
27 - Crystal Shard 1/3
28 - Cameo 8/10 - Dask
05:04 - Episode 12 - It Won't Fly!
29 - Disgusting Bug 8/8
05:15 - Episode 13 - Technodrome Redux
30 - VHS Tape 5/5
31 - Cameo 9/10 - Kala
32 - Crystal Shard 2/3
05:44 - Episode 14 - The Lost Archenemies
33 - Cameo 10/10 - Zak
06:02 - Episode 15 - Outworld Stangeoids
34 - Crystal Shard 3/3