However, as noted here, Leibl himself was greatly influenced by Gustave Courbet. One possible result of this was his usual practice of composing paintings on the fly rather than doing a lot of careful preparatory work as academicians would do. (James Gurney had a short post about Leibl's technique here.)
This did not prevent Leibl from painting a subject more than once. So while each work might have been done extemporaneously, collectively they might be considered studies. Examples of his depiction of one subject are included below.
In general, his paintings as found on the Internet tend to be free, sketchy. But he was quite capable of working in a more precise manner.
This seems to be Leibl's most famous painting. It is carefully done ... not spontaneous.
This early portrait is sketchy indeed. Might it actually be a study?
Another carefully-done work.
Whereas this portrait of the same subject seems to be a study: note, for example, an alternative right arm and the apparent lack of a signature.
No date on this Sketch. She seems to be the subject of the following two or three images -- and quite possibly the nearest of the three women in church shown above.
That is the title attached to the image as I found it on the Web, but it's clearly Maria Ebersberger.
Again, the Internet title -- but also Maria (compare the ear as well as other features to the images above).
The Internet date is 1898, whereas the above images of Maria Ebersberger seem to be from around 1880. Nevertheless, this seems to be Maria shown at about the same age.
A later portrait sketch.
And another of Leibl's later paintings.