If several people have contributed as authors to the creation of a work, they are called joint authors. These joint authors all have a joint copyright in the work (Art. 7 para. 1 CopA).
The joint authors can create a joint work by collectively working on an idea or objective (e.g. several artists create a sculpture together), or each joint author can create their own work which is then added to the joint work, together with the work of all the other joint authors (e.g. several writers write a text contribution to a joint book). In both cases, it is important that everybody gives creative input.
Unless agreed otherwise, the joint authors may only use the work with the consent of all the authors. The other joint authors may not arbitrarily refuse to give their consent ('contrary to the principles of good faith') (Art. 7 para. 2 CopA) and usually have to give their consent if the work is used for its customary purpose. If the individual contributions can be separated and there is no agreement to the contrary, each of the joint authors may individually use their own contribution, provided that this does not affect the exploitation of the joint work (Art. 7 para. 4 CopA).
A distinction must be made between the joint author and the executing person. The latter only follows instructions and does not give any creative input. An executing person therefore does not have any copyrights. (E.g. if an employer makes use of its authority to give directives and tells an employee exactly how to create a work; the employee does not give any creative input.)
It can also be difficult to determine joint authorship when an existing work is combined with a new work (derivative works, Art. 3 para. 1 CopA or collected works, Art. 4 para. 1 CopA).
In this case, the author of the existing work and the author of the new work are not joint authors. The criterion for joint authorship is that a work must be jointly created; this is not fulfilled here.
Both of them are joint authors. They are engaged in a joint creative task – in this case, the creation of a children's book – and are jointly pursuing this objective.
No, the lecturer is the only author. The assistant created the slides as directed by the lecturer and did not give any creative input of their own. The assistant is just the executing person.
The lecturer and the assistant are joint authors. They have a joint task, i.e. to add the contents of the lecture to a PowerPoint presentation; both the lecturer and the assistant give creative input.