I only want it to be called manually.

the packages may have a postinstall script, which will be automatically started after copying the files. Without that the installation is incomplete.
So if you install a new kernel grub-install will be automatically executed.
I don't know your case, if it is similar or not, you gave no any details.

That is my case where I need grub-install not to be called automatically. Which details should I please post ?

Is this during installation or an installed system?

an installed system

The case we know nothing about.
All we know is that grub-install was (automatically?) invoked, which you don't want.

What are you trying to do at all?
What do you want to achieve (it is not enough to say what you don't want)?
What commands did you execute, what's happened?
What OS and environment do you have?
Why do you want to skip it?
http://www.catb.org/~esr/faqs/smart-...html#beprecise

This is about 5 questions, a 3 word answer will not be enough.

grub-install is automatically invoked after some upgrades which I don't want.

I use multiboot with openSUSE Tumbleweed on btrfs which Debian's grub can't access (result is that TW is not visible in Debian's grub menu) so I boot Debian using TW's grub. I want use TW's grub only to boot both distros (and they are usually both visible in TW's grub).

I did not try any command so far.

Both Debian and TW are 32bit and use IceWM.

Because TW is not visible in Debian's grub menu.

you don't need two grubs, one is enough and it should detect both OSs. At least I think that would be the optimal solution.
You can implement a dummy grub-install script and replace the original, temporarily, if you wish.
It is not a good solution and may cause additional troubles, but anyway you can try it.

I'd be surprised that grub-install is run on any updates. The grub-mkconfig script is run by some systems when a new kernel is installed. If you could, post which of the two systems does this, is it Debian or Opensuse? Are both of these systems Legacy/CSM installs and, if so is Opensuse in the MBR of the drive? Are both Debian and Opensuse on the same hard drive? In any case, you want to use the Opensuse Grub then make sure it has code in the MBR if a Legacy install or if EFI, it is set to first boot in the BIOS or if they are on separate drives, set the drive with Opensuse to first boot in the BIOS.

Uninstall the grub package that does grub-install, not sure which one, or uninstall all of the grub packages. Mark to hold the grub packages not to upgrade.

Only one of the installed OS' should host the grub installation in any multi-boot scenario. Elect one OS for this purpose and remove grub entirely from all others.

This should have the desired result:

Then reboot into the OS which hosts the grub install and set up from there.