| Name | Character | Episodes |
|---|---|---|
| Paul Vaughan | Narrator (voice) | 6 |
| Alice Roberts | Self - Presenter | 4 |
| Joss Ackland | Narrator (voice) | 3 |
| Hannah Fry | Self - Presenter | 3 |
| Brian Cox | Presenter | 3 |
| Helen McCrory | Narrator | 2 |
| Michio Kaku | 2 | |
| Richard Feynman | Self | 2 |
| Richard Feynman | Himself | 2 |
| Kelly Hunter | Herself - Narrator | 2 |
| Helen Czerski | Self - Presenter | 2 |
| Xand van Tulleken | Presenter | 2 |
| Steven Mackintosh | Himself - Narrator | 2 |
| Jeff Goldblum | James Watson | 1 |
| Toby Jones | Narrator | 1 |
| John Hannah | Himself - Narrator | 1 |
| Ian Holm | Narrator | 1 |
| Andrew Scott | Narrator | 1 |
| Lyndon Brook | Erwin Chargaff | 1 |
| Jimmy Carr | Presenter | 1 |
| Alan Howard | Maurice Wilkins | 1 |
| Carl Sagan | Self | 1 |
| Katherine Parkinson | Narrator | 1 |
| Adam Pearson | Presenter | 1 |
| Rupert Penry-Jones | Himself - Narrator | 1 |
| Juliet Stevenson | Rosalind Franklin | 1 |
| Jemma Redgrave | Narrator (voice) | 1 |
| Hugo Speer | Narrator | 1 |
| David Attenborough | 1 | |
| David Attenborough | Self | 1 |
| David Attenborough | Himself / Narrator | 1 |
| Paul McGann | Himself | 1 |
| Adrian Lester | Himself - Narrator | 1 |
| Arthur C. Clarke | 1 | |
| Stephen Hawking | Himself | 1 |
| James Hazeldine | Self | 1 |
| Robert Winston | 1 | |
| Nicholas Fry | Raymond Gosling | 1 |
| Bernard Hill | Narrator | 1 |
| David Harewood | Narrator | 1 |
| Betsy Brantley | Elizabeth Watson | 1 |
| Lucian Msamati | Narrator | 1 |
| Tim Pigott-Smith | Francis Crick | 1 |
| Terrence Hardiman | Trofim Lysenko | 1 |
| Bill Paterson | 1 | |
| Hannah Fry | Self | 1 |
| Thomas A. Edison | Self | 1 |
| Rosemary Leach | Self - Narrator (voice) | 1 |
| Brian Cox | Personter | 1 |
| Jim Al-Khalili | Self - Presenter | 1 |
| Liz Bonnin | Self - Presenter | 1 |
| Liz Bonnin | Herself - Presenter | 1 |
| Richard Branson | 1 | |
| Jane Goodall | Self | 1 |
| Julian Assange | Himself - Interviewed Guest | 1 |
| John Money | Self | 1 |
| Xand van Tulleken | Self - Presenter | 1 |
| Steven Mackintosh | Narrator | 1 |